Imaging technologies have been developed for the visualization of the interior of an internal channel, such as a vein or artery. For example, current Optical Coherence Tomography (OCT) systems can image into tissue to a depth of about 1.2-1.7 mm. Analogous to ultrasound imaging, the imaging core of an OCT system projects an optical beam (e.g., a short coherence length infrared light) on the tissue and receives the reflected light from the tissue to construct an image of the tissue. An OCT based imaging system can provide higher resolution imaging than current ultrasonic systems, but to a shorter depth into the tissue.
FIG. 1 illustrates a prior art imaging assembly catheter. In FIG. 1, the imaging assembly includes a sheath (109) to house and guide the imaging core, which includes an optical fiber (107), a GRIN lens (103) (graded index lens) and a prism (101). The sheath (109) has at least a transparent window (105), through which an imaging core may project an optical beam on the tissue and receive the backscattered light. The GRIN lens (103) focuses the beam coming from an optical fiber (107); and the prism (101) directs the beam perpendicular to the longitudinal axis of the imaging sheath (109). Backscattered/reflected light from the tissues follows the reverse path to return to the optical fiber (107). Optical fibers have a core and a cladding. The light travels mostly in the core. The cladding has optical properties such that it bends any light that happens to come out of the core back into the core, so no light is lost/leaks out the side of the optical fiber. In practical systems for OCT, both the core and cladding are mostly glass with a few impurities added to get the desired optical properties. The interference amplitude between an internal system reference beam and the reflected beam from the tissue is related to the intensity of the reflected light from the tissue at the reference beam path length. By scanning the reference beam path length, the amplitude of the reflected beam is scanned at different depths into the tissue to create an image along a line into the tissue. By also rotating (113) the imaging core, a two-dimensional image slice of the tissue surrounding the prism (101) can be constructed. By withdrawing the imaging core inside the sheath (or both the sheath 109 and the imaging core), image slices of the tissue may be recorded along a length of the sheath 109 (or tissue) to provide three-dimensional tissue imaging information. This withdrawal to gather tissue images is commonly referred to as a pull back.
FIG. 2 illustrates the use of the prior art imaging assembly in scanning the interior wall of an artery. The sheath (109) is inserted into an artery (121) with the imaging core, including the grin lens (103) and prism (101). The prism directs the optical beam to a point on the artery to scan at different depths into the tissue at the point. Rotating the imaging core about the axial axis of the sheath (109) causes a scan in the circumferential direction; and moving the imaging core along the sheath (109) causes a scan in the longitudinal direction. Thus, the combined rotation and longitudinal movement of the imaging core allows the imaging core to scan the entire interior of the artery (121) for a 360-degree of circumferential image of the artery.
In a prior art system illustrated in FIG. 2, the sheath (109) is substantially straight in a straight section of the artery. Note that an artery is generally not straight; and the sheath generally follows the artery. For simplicity, the artery (or the vessel to be image) is considered straight and the sheath that follows the shape of the artery is then also considered straight. When the sheath (109) is at about the center of the artery, the distance between the sheath (109) and the artery wall is DA (123).
Current Optical Coherence Tomography (OCT) systems are not able to image more than about 1-2 mm into blood or tissue. Thus, the imaging depth of the OCT system is only about 1-2 mm. Theoretically, an OCT system may be able to image about 2-2.5 mm into tissue; but in practice, in the tissues of interest (vessel walls), a typical OCT system can image no deeper than about 1.2-1.7 mm. Thus, the blood between the artery (121) and the sheath (109) can effectively cause blockage of the light signal for the imaging of the artery wall and reduce the depth the OCT system can image into the artery wall.
Further, the wavelength of the light used in an OCT system may be short enough for the light to interact with individual red blood cells. Use of lights of longer wavelengths may avoid the red blood cell interaction but result in a loss of a desired image resolution. The red blood cells have a slightly higher index of refraction than the plasma in which they are suspended. In addition, the red blood cells are shaped like concave lenses so that the light may be redirected and refocused (diverged) by each red cell it passes through on the way to the artery wall and back from the artery wall. In addition, there are light energy losses due to absorption and path length changes due to scattering (reflection) by the red cells. Thus, the image quality decreases as the distance DA (123) between the sheath and the artery increases. It is desirable to minimize the effect of the blood's interference with the light from the imaging system as it propagates through the vessel towards the vessel wall and is reflected back to the device.
Currently, flushing is used to reduce the imaging signal (light) blockage effect of the blood. For example, saline can be injected into the artery from the catheter to temporarily remove or dilute the blood near the region to be imaged. Various techniques and devices have been used to flush blood from the imaging area with limited success. For example, flushing a coronary artery to remove blood from the field of view is normally accomplished by injecting saline into the vessel to be imaged, either through a guide catheter or a catheter/sheath that surrounds/incorporates the imaging device. However there are several problems and limitations with flushing, especially in an artery.
First, when enough saline solution or other isotonic biocompatible water-based solution is introduced to replace or dilute the blood, the amount of oxygen in the solution is very small in comparison to the amount of oxygen contained in the blood. Thus, the time window for imaging is limited by the ischemic consequences of the solution on the heart muscle (e.g., reduction in blood flow). The longer the duration of the flush, the more severe the consequences are to the heart muscle. Since imaging is generally desired in patients usually already suffering from ischemia or previous cardiac muscle ischemic tissue damage, the safe/pain-free imaging time period is short.
Second, blood flow in coronary arteries is laminar and generally tends to flow in streamlines, not mixing very rapidly with adjacent streamlines. Thus, injected solutions tend to flow in their own streamlines, leaving some areas of blood flow not completely displaced/mixed or leaving eddies of blood at branch points or at areas protected/created by the presence of the imaging device.
Third, most water-based flushing solutions have a viscosity that is significantly less than that of blood. Thus, the flow rate of the flush must exceed the normal flow rate of the blood in the vessel in order to create enough pressure in the vessel to exceed the blood pressure and displace the blood. In other words, the resistance to flow in the vessel is lower for the flush than for the blood.
As the flush replaces the flowing blood, an ever-increasing flow rate of the flush is required. For example, the decreased resistance of the flush requires more overall fluid (e.g., flush) to maintain the natural flow rate. Moreover, the vessel will dilate in response to the ischemic properties caused by an increased amount of oxygen deficient fluid in the vessel. Thus, the flush flow rate must be increased until a peak flow rate is reached, wherein the flush effectively completely replaces the blood in the artery. The volume of flush required to achieve this peak flow rate can be quite high during extended imaging periods, like those commonly used with IVUS (Intravascular Ultrasound).
Fourth, in most injection configurations, the required high flush flow rate enters the artery via a relatively small flow cross section, resulting in a very high injection velocity. This may create high velocity jets of flush, which can damage vessel walls. Additionally, the pressures and volumes required are not easily accomplished by manual injection. Therefore, an automated injection device is desirable.
Alternatively, injection of a fluid more viscous than saline (e.g., a contrast agent) may utilize a lower flow rate, but the catheter injection pressure is relatively unchanged due to the higher viscosity. A high viscosity flush also increases the time required to wash out the flush (e.g., longer ischemia time). Moreover, contrast agents are quite expensive relative to normal flushing solutions.
Several methods to deal with these problems of a typical flush have been proposed in the past. For example, oxygenated blood can be withdrawn from the patient, and certain materials may be added to the blood to increase the index of refraction of its plasma to match that of the red blood cells. This oxygenated blood, with a higher index of refraction of its plasma, can then be used as the flush. Alternatively, the materials to increase the index of refraction of the plasma may be added systemically without withdrawing any blood from the patient.
In either case, such a procedure would eliminate/effectively minimize the lens effect and the reflection effect of the red blood cells. Since the red blood cells are oxygenated, ischemia is not a problem. It has been reported that contrast can be used to make this index of refraction change to the plasma.
Changing the index of refraction on a systemic level is very difficult and can be toxic. It is easier and faster to perform the index of refraction change with blood withdrawn from the body. However, changing the index of refraction outside of the patient's body requires extra equipment and a time-consuming index matching procedure and introduces issues involving increased blood exposure (e.g., to the environment). Moreover, the streamline and injection problems discussed above would still be a challenge, and hemolysis (e.g., the destruction or dissolution of red blood cells, with subsequent release of hemoglobin) could be an added issue to consider.
Photodynamic therapy may be administered within a vessel to treat various conditions. For example, light (e.g., blue light and/or ultraviolet light) may be used to destroy (e.g., cell lysis) or treat various target tissues such as tumors and atheromas, including thin capped fibroathroma (“TCFA”) or vulnerable plaque. A similar blockage of the light used in photodynamic therapies may also be a problem and may require similar saline flushing or blood dilution.